This is an ongoing project headed by Dr. DiMaio. Senescence is an important tumor suppressor mechanism. Not only does senescence appear to prevent tumor formation in vivo, but it may be possible to mobilize a durable senescence response as a new approach to treat cancer. With the support of this grant, we have developed a new model of cellular senescence induced by transcriptional repression of the human papillomavirus oncogenes in cervical carcinoma cell lines. In contrast to most other models of senescence, senescence induced by HPV repression is rapid, uniform, and synchronous. We will use this model to explore two important aspects of senescence. First, we will determine the molecular mechanism by which the initial growth arrest caused by HPV E7 repression is converted to an irreversible senescent state. We will test the hypothesis that the stable assembly of heterochromatin at repressed promoters is the primary cause of irreversibility. We will use this system to determine how this ~epressive heterochromatin is formed, and we will conduct a genetic screen to identify cellular genes with the ability to either reverse senescence on their own or to cooperate with re-expressed viral oncogenes to reverse senescence. Second, we will conduct a comprehensive genetic and biochemical analysis of the role of cellular microRNAs in determining the senescence phenotype and the pattern of cellular gene expression in senescent cells. In collaboration with Dr. Steitz, we will compare microRNAs activity in two different growth-arrested states, senescence and quiescence. Taken together, these experiments will provide new insights into the molecular mechanisms that establish and maintain the senescent state, and may set the stage for attempts to translate this knowledge into rational new approaches to treat or prevent cancer by modulating the senescence response.